Japanese Patent Laid-Open Publication No. 3-96469 discloses a hydraulic braking pressure control system for a vehicle having a motor failure detection system. The system includes a hydraulic pressure generating apparatus which generates a pressure in response to brake pedal operation, electromagnetic hydraulic pressure control valves which control the hydraulic pressure generated by the hydraulic pressure generating apparatus, a brake cylinder which applies a braking force to the wheels, a reservoir which is connected to the brake cylinder, a pump which sucks brake fluid from the reservoir and supplies brake fluid to the hydraulic pressure generating apparatus, and a motor which drives the pump.
In this system, a pressure control system detects the operating condition of the reservoir and the motor. The pressure control system has a first motor drive device, a second motor drive device, a first condition check device and a second condition check device. The first motor drive device drives the motor for a predetermined time period so the motor rotates at a constant speed. In this system, the time is set at 250 ms. The first condition check device detects the output voltage of the motor after the motor has stopped. Based on the output voltage of the motor, the control system determines if the system has a failure, i.e., if the pump has become stuck or if a fluid leakage to the reservoir has occurred. If the output voltage of the motor, when a predetermined period of time has passed after the motor has stopped, is greater than a predetermined value, the control system determines that there is no failure of the motor or the reservoir. If the output voltage of the motor is smaller than the predetermined value, the system determines that there is either a failure of the pump or a fluid leakage. The predetermined period of time after the motor has stopped is set at a time shorter than a time the output voltage of the motor drops to zero when there is no failure of the pump or the reservoir after the motor reaches the constant speed. The predetermined period of time is set to be a time longer than a time the output voltage of the motor drops to zero when there is a failure of at least the pump or the reservoir.
The second motor drive device drives the motor for a second predetermined period of time which is longer than the first predetermined period of time so the fluid in the reservoir can be sucked completely if the reservoir is full of fluid. The second condition check device determines if sticking of the pump occurs based on the output voltage of the motor after the second motor drive device drives the motor. If the output voltage of the motor, when a predetermined period of time has passed after the motor has stopped, is greater than a predetermined value, the control system determines that the reservoir has a leakage. If the output voltage of the motor is smaller than the predetermined value, the system determines that a pump stick has occurred.
However, the system described above suffers from a variety of drawbacks and disadvantages. In one respect, The system determines a failure according to the output voltage drop after the motor reaches a constant speed. As a result, the motor drive period becomes longer and motor operating noise lasts longer.
In addition, if the system stops the motor before the motor reaches a constant speed, the output voltage of the motor becomes smaller. As a result, the time it takes the output voltage of the motor to drop to zero becomes too short so the system cannot accurately detect a failure.
The system also does not take into account the value of the operating pressure. If the pressure is near zero even when brake fluid still remains in the reservoir, the output voltage of the motor drops more slowly than when the pressure is high. The system may thus make an error that there is no failure.